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Re: When does "live birth" become a critical survival edge?



At 11:03 AM 11/12/97 -0800, Alan Sinder wrote:

>As a layperson, I find these questions fascinating. The fast furry
>reptiles
>that went on to sweat and milk and live birth aren't the same type of
>reptiles
>that evolved into the theropod-like dinosaurs and birds. So what factors
>would
>cause those needs. Aren't the hearts of mammals four chambered and birds
>three
>chambered? 

Actually, birds have four-chambered hearts, too, and crocs have a modified
four-chambered heart.  The "plumbing" of archosaur hearts and of mammal
hearts, though, is different.

Also, mammals are not the descendants of reptiles (at least not as that term
is currently used).  "Reptilia" no longer means "an amniotic egg-laying
cold-blooded vertebrate" or even "any amniote unlucky enough not to have
evolved into a bird or a mammal".  Reptilia is defined as all descendants of
the most recent common ancestor of Testudines (turtles), Lepidosauria
(tuataras and lizards, including snakes), and Archosauria (crocodiles and
birds and their ancestors).  These groups are united by many hard and soft
tissue features, as recognized last century by Thomas Huxley and others.

Mammals, however, do not descend from the most recent common ancestor of all
these guys.  Instead, the mammals trace their ancestry to a different branch
of the Amniota (hard egg-shell layers). Mammals and their ancestors
(together called the "Synapsida") lacked the specializations found in true
reptiles.  For example, mammals retain the primitive condition of a highly
glandular skin: our skin houses many sorts of specialized glands (in our
case, sweat, mammary, pheremone, etc.).  On the other hand, reptiles lack
highly glandular skin.

Lastly, evolution isn't about "needs" as such.  It is about the differential
survival of variants in a population.  In all population there is some
variability, and some of that variability is genetic.  More offspring are
produced than can possibly survive.  Given these circumstances, some
variations will have a better-than-average chance of surviving and getting
passed on to the next generation.  Over a long enough time, this can produce
profound changes.

So, it isn't necessarily a case that that synapsids "needed" to produce milk
and live birth.  Instead, if a variant had sweat glands that produced extra
salts or a variant that retained the egg in the body longer had an
advantage, it might have a better than average chance on surviving to
breeding age and passing on those traits.

(Or, as R. Dawkins replies to the question "What good is half an eye?":
"It's better than no eye")

>I am curious as to when it becomes a bonus to invest time and energy in
>rearing
>offspring as well.
>
This is a classic situation in ecology: is it more effective to produce lots
and lots of young, on the chance that enough will survive to adulthood; or
to produce a few young in which more energy and time are invested, again on
the chance that such investment will insure that enough youngsters actually
make it to breeding age.  Both stragies are found in nature, so it isn't
necessarily that one is better than another.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661